The present invention relates to a method of and apparatus for manufacturing electric wires having a coating of synthetic resin, wherein a synthetic resinous material, such as polyethylene, having a cross linking agent, such as an organic peroxide mixed therein, is extruded onto a electrically conductive wire and is subsequently subjected to a cross linking step to harden the resin.
High pressure steam has been commonly used to effect the cross linking step of a method of coating a wire with a synthetic resinous material having a cross linking agent, such as an organic peroxide, mixed therein. In some cases inactive gases of high pressure and temperature are used instead of the high pressure steam, for increasing the efficiency and elevating the quality of the product.
These known methods are inconvenient in that a very large apparatus is required to withstand the high pressure steam or gas. For example, steel pipes, which can withstand pressures of approximately 10 to 20 Kg/cm.sup.2, are used as the reaction pipes within which the resin is hardened. In order to overcome this shortcoming, it has been proposed to directly connect the extruder for extruding the coating material onto the wire to a metal pipe in which the cross linking reaction is expected to be performed. In this method, a suitable lubricant is supplied into the reaction pipe, as well as into the extruding die, so that the extruded resinous material, partially hardened, can flow continuously into the pipe until the cross linking reaction is completed by the heat supplied from the outside of the pipe. The latter method is also unacceptable in that the length over which lubrication is required is very long, because a considerably long time is required for the synthetic resinous material to be cross linked and hardened completely by the heat delivered from the outside of the pipe so that the material must travel a correspondingly long distance.
Thus, the latter method suffers the following shortcomings:
(1) The equipment is uneconomically large. PA1 (2) Complicated equipment is required for effecting uniform lubrication over the entire length of the long metal pipe. PA1 (3) Considerable time and effort are required for replacing the die and the long metal pipe with another assembly, when a product of a different size is required. PA1 (4) It is difficult to maintain the wire material coaxial with the coating layer, i.e. to obtain a concentric product, during the travel in the long pipe, especially when a heavy conductive body is to be coated by relatively thin coat layer.
The shortcomings of the prior art will become clearer from the following description of a longitudinal section of a prior art apparatus for effecting the cross linking reaction with a lubricated metal pipe, as shown in FIG. 1. The synthetic resinous material with a cross linking agent is kept at a temperature below the decomposition temperature of the agent, until it is fed to an extruding die 31. Therefore the resin is maintained in a fluid state. For example, at a point designated at A, the resinous material is kept at a temperature below 130.degree. F. As the material advances into a pipe 33 for effecting the cross linking operation, a lubricant is supplied from a metal pipe lubricator 32. The material is gradually hardened as the reaction takes place by the heat transferred from a heater 34 through the pipe 33 and the lubricating oil film. The pipe 33 is kept at a temperature between 200.degree. and 350.degree. C. In this process, the speed of processing depends on the heat transfer, and it takes a considerable time for the resinous material extruded at a temperature lower than 130.degree. C. to be evenly heated up to some 200.degree. C. at which the reaction takes place, because the synthetic resinous material usually has poor heat conductivity. This means that the distance between the point A and a point B, at which cross linking is completed, is inevitably long and a correspondingly long metal pipe is required. For example, a metal pipe 33 would have to be 15 to 20 meters long to fabricate a 70 mm diameter wire with a coating which is 20 to 30 mm thick.